1. Field of the Invention
The present invention pertains to liquid distributors for vapor-liquid contact towers and, more particularly, to a flow equalized liquid distributor for columns incorporating counter-current, vapor-liquid flow therethrough.
2. History of the Prior Art
It is well known in the prior art to utilize various types of exchange columns in which a gas and a liquid come into contact with one another, preferably in a counter-current flow for purposes of mass or heat transfer, close fractionation and/or separation of feed stock constituents, and other unit operations. Efficient operation requires mass transfer, heat transfer, fluid vaporization and/or condensation, whereby one of the fluids can be cooled with a minimum pressure drop through and in a particular zone or zones of minimum dimensions defining the area and volume thereof. These are prerequisites of efficient operation and are necessary for close fractionation. For this reason counter-current flow of vapor and liquid within such exchange columns have become established methods of such vapor-liquid contact in the prior art. The actual vapor-liquid interface requires the utilization of a packing bed within the column. Liquid is then distributed atop the packing bed in the most feasible manner while vapor is distributed beneath the packing bed in the lower region of the tower. In this manner liquid trickling downwardly through the packing bed is exposed to the vapor ascending therethrough for vapor-liquid contact and interaction.
It is well established that the configuration of the packing bed determines the pressure drop, efficiency of the vapor-liquid interface and the concomitant mass and energy transfer occurring in a process tower. The means for effective and even distribution of the vapor and the liquid on opposite ends of the packing bed as well as maintenance of that distribution therethrough are critical to an efficient operation. Only with efficient initial vapor and liquid distribution and the maintenance of said distribution throughout the packing bed, will homogenous mixing zones be created therethrough for maximizing the efficiency therein. Efficiency is readily convertible to cost of operation and production quality. For this reason, a myriad of prior art packing designs have been prevalent in conventional exchange columns. The efficiency of the packing is, however, limited to a large extent by the efficiency of the vapor and liquid distribution thereacross. For example, failure of either vapor or liquid to evenly distribute over cross-sections of the packing effectively eliminates the utility of the part of the packing where there is poor or no distribution which in turn is directly proportional to the efficiency and cost effectiveness of the operation. Packing bed depths are critical in establishing production criteria and operational costs and failure to evenly distribute vapor-liquid and/or maintain homogeniety within the packing bed can lead to serious consequences, particularly in the petroleum refining industry.
Aside from the packing beds themselves, the liquid distributor is the most important unit of a tower internal. Failure in performance of a packed tower sometimes stems from liquid distribution problems such as clogging or uneven distribution, and thus the selection of a correct liquid distributor is critical for uninterrupted plant operation. Operational considerations thus include the functional aspects of the distributor, such as how level the distributor troughs are maintained, how well the flow is equalized therethrough, and the means through which the liquid is distributed from said troughs to the packing beds therebeneath. Also considered is the effect which the ascending vapor has on the liquid being distributed. When vapor flow areas are restricted, flow velocity can increase to the point of interrupting the descending liquid flow pattern. The liquid is, in essence, "blown" around, and this conditioning can lead to uneven distribution and inefficiency.
Conventional liquid distributors generally include the multi-orifice spray head variety adapted for dispersing liquid in the form of a spray atop a packing bed. In the utilization of dump packing wherein a plurality of randomly oriented packing elements are disposed within the exchange column, such a liquid distribution technique is sometimes effective. This is true particularly when high efficiency parameters are not of critical significance. However, in the event of high efficiency packing such as that set forth in U.S. Pat. No. 4,604,247 assigned to the assignee of the present invention, means for homogeneous liquid and gas distribution are of extreme importance.
The cost of high efficiency packing of the type set forth in the aforesaid patent commands attention to proper vapor-liquid distribution. Even small regions of non-homogenous interaction between the vapor and liquid is an expensive and wasteful loss not consistent with the utilization of high efficiency packing, where space and homogeniety in vapor-liquid interface is both expected and necessary for proper operation. High efficiency packing of the state of the art varieties set forth and shown in the aforesaid U.S. Patent requires counter-current vapor-liquid flow through the channels defined by opposed corrugations of sheets disposed therein. If the initial liquid or gas distribution fails to enter a particular corrugation pattern, then precious surface area is lost in the packing until the liquid and vapor are urged to migrate into and interact through the unfilled regions of the packing. Only by utilizing proper vapor and liquid distribution means may effective and efficient utilization of high efficiency packing as well as conventional dumped packing be assured.
The development of systems for adequate liquid distribution in process towers has been limited as set forth above. In the main, it is known to discharge and distribute liquids with spray orifices, pipes, perforated plates, apertured troughs and nozzles. Gas is concomitantly discharged in an ascending turbulent configuration to provide adequate vapor distribution. Although many prior art systems are generally effective in distributing some vapor and some liquid to most portions of the packing bed, uniform distribution thereacross is usually not obtained without more sophisticated distribution apparatus. For example, unless gas is injected into a myriad of contiguous areas beneath the packing bed with equal pressure in each area, the mass flow of vapor upwardly through the packing bed cannot be uniform. Random vapor discharge simply distributes unequal amounts of vapor across the lower regions of the packing bed but does not in any way assure equality in the distribution. Likewise the simple spray of liquid atop the packing bed, though intended to be effective in wetting all surface areas, often results in high concentrations of liquid flow in certain packing bed areas and less flow in others. This, of course, depends on the spray device. Orifice distributors are generally more susceptible to plugging than other types of distributors, and plugging is generally non-uniform leading to uneven irrigation within the tower. Surface irregularities in a distributor pan occurring during manufacture likewise increase flow resistance of some perforations or induce liquid flow along the bottom of the pan which is a distinct disadvantage. Any flow irregularity which focuses the flow in one area while reducing flow in other areas is deleterious.
It has been discovered that with pipe distributors consisting of headers equipped with tributary pipes or laterals that have holes or nozzles to spray liquid, the liquid is often distributed too finely. Tiny drops of the liquid then get carried out of the tower by counter-current gas flow. This prevents the liquid from even coming in contact with the packing bed. Since liquid contact is the purpose of the packing therebeneath, such a result totally frustrates the intent of the liquid distributor. As much as 5% of the liquid flowing through a nozzle can be converted to mist at a pressure drop of 20 psi. It has also been noted that nozzle equipped pipe distributors can produce overlapping spray patterns which result in increased flow in certain areas with reduced flow in other areas. Moreover, spray headers also release liquid at speeds that can cause it to pass vertically through the packing before it has a chance to spread out horizontally depending on the particular packing type.
These issues are important as well as the critical issue of the number of liquid distribution points necessary for various tower diameters, packings, heights, materials and systems. It is critical that the packing height not be too great wherein the weight of the packing will cause it to crush itself. However, liquid redistributors between packing sections are expensive and take up heights that could otherwise be used for mass transfer. One consideration is the type of packing being used. Structured packing can tolerate very little maldistribution while dump packing on the other hand can sustain larger variations in liquid distribution.
Unfortunately, the manifestation of uneven liquid distribution generally occurs in the vicinity of the most even, or uniform, vapor distribution. The opposite is also true. This is because vapor has had a chance to more evenly distribute through the packing bed prior to engaging the liquid distribution flow. It would be an advantage, therefore, to provide means for even liquid and vapor distribution prior to entry of said vapor and liquid into the packing bed and in a manner providing both a uniform spread of said liquid and vapor and uniform volumetric distribution thereof.
The present invention provides such an improved system of vapor-liquid distribution through a tapered trough, high efficiency distributor wherein each trough maintains a uniform liquid level through a flow equalizing system. The system is provided with an interconnected series of flow pipes that flow into the lowermost portions of the narrow, tapered troughs to equalize liquid levels and allow for drainage of liquid and debris therefrom. The flow pipe facilitates movement of multiphase liquid through the network of open, tapered troughs equipped with discharge notches or holes which would otherwise malfunction if not perfectly level or clog if debris could accumulate. Angled baffles disposed outwardly of the tapered troughs prevent liquid from being "blown" around by the ascending vapor as well as uniformly dispensing the flow. Liquid spreading screens or wires can also be used with the baffles. Pressure drop is minimized through the distributor by maximizing the free area across the tower internals. Liquid flow efficiency is thus provided in a more cost efficient, reliable configuration.